1. Field of the Invention
This invention relates to analog to digital conversion and more particularly to a method and apparatus for use with analog to digital converters of the sigma-delta type.
2. Description of the Prior Art
The sigma-delta modulator has been known since 1962. However, it was not until the recent advances in integrated circuit technology that related techniques became cost effective for use in high precision analog to digital converters of the sigma-delta type. The sigma-delta type converter typically consists of digital circuitry with a small amount of analog circuitry.
Existing techniques use the concepts of noise shaping, oversampling rate, and decimation to achieve the performance seen in the known sigma-delta type analog to digital converters. Some of these known converters are designed for use in the audio market which requires, for example, 16 bit resolution of a 20 KiloHertz signal. However, in industrial control applications the requirement can be to convert a DC signal to a few Hertz analog signal with greater than 16 bit resolution.
A good reference which describes the functionality of a sigma-delta modulator for use in analog to digital conversion is "A Sigma-Delta Modulator as an A/D Converter", by Rudy J. van de Plassche, IEEE Transactions on Circuits and Systems, Vol. CAS-25, No. 7, July 1978.
A sigma-delta converter quantizes an incoming analog signal with one (1) bit resolution using "oversampling", that is, an extremely high sampling rate relative to the highest frequency of interest of the incoming analog signal. The term "oversampling" refers to the relationship between the frequency or sampling rate at which the converter samples as compared to the well known Nyquist rate. The Nyquist rule states that the sampling rate must be at least twice the maximum frequency of the measured analog signal in order to prevent aliasing effects.
Existing methods often use decimation filter techniques in the digital filtering section of a sigma-delta analog to digital converter. Several decimation techniques are described in "A Comparison of Decimation Filter Architectures for Sigma-Delta A/D Converters", by Peter Y. K. Cheung, Eric S. K. See, 1991 IEEE International Symposium on Circuits and Systems, Vol. 3 of 5, Analog, Circuits and Neural Networks, Singapore, 11-14 Jun. 1991, pp. 1637-1640.
One such decimation technique is to use a comb filter. At first glance the use of a decimation comb filter appears to yield substantial benefits, because it is a technique which allows a one (1) bit stream at 640 KHz for example, to construct 12 bits at 40 KHz. This seems impressive considering that the resolution increases by a factor of 4096 while the frequency drops by a factor of only 16.
Allowing the comb filter to run continuously does provide improved performance because it acts as one huge moving average. However, there are problems associated with the use of a decimation comb filter. One such problem, as those skilled in the art will appreciate, is that a decimation comb filter usually requires additional digital filtering to compensate for out of band noise. Another such problem is associated with the mechanism used by the comb filter to accomplish the decimation filtering. This mechanism requires that either only one analog input channel is connected to a sigma-delta converter or, if the comb filter serves more than one channel, the necessity to store the data related to each channel in order to maintain continuous operation of the decimation comb technique as analog input channels are alternated. The amount of storage required will depend on the complexity of the comb filter and the number of channels served by the filter.